This invention relates generally to electro-optical scanning systems adapted to sense a bar-code pattern and to generate digital signals corresponding thereto, and more particularly to a highly-compact scanning unit employing a luminous scanning line derived from an incandescent light source.
In order to facilitate the sale of packaged goods and other consumer items, for purposes of inventory control and to expedite various transactions in which it is necessary to identify the items being processed by a computer terminal, it is now the common practice to print on each item a bar code having a unique pattern. Thus every item of a particular kind bears a common symbol which distinguishes it from different items.
The typical bar code, such as the Universal Product Code (UPC), employed in a point-of-sale merchandising system to expedite supermarket checkout transactions, takes the form of a series of parallel bars whose varying widths and spacings characterize the identifying pattern.
To recover the information provided by a bar-code pattern, use is made of an electro-optical scanner. In scanners of the type heretofore known, whether in elementary form or in a more sophisticated construction, scanning is effected by means of a luminous spot derived from a light source, the spot being manually or mechanically swept across the bars of the printed code and being reflected thereby to produce modulated light pulses which are intercepted by a photodetector whose output yields digital signals corresponding to the bar code.
One simple and well-known electro-optical scanner is constituted by a two-way fiber optic wand which is manipulated by an operator to sweep a luminous spot across the bar code. The wand is coupled by a fiber optic cable to a light source and to a photodetector, whereby a beam of light from the source is transmitted by the cable to the wand and projected therefrom onto the bar code, the reflected light pulses being fed by the cable to the photodetector.
The ability of a hand-held fiber optic wand to provide a correct read-out of a bar code depends, of course, on the care exercised by the operator. But apart from the obvious limitations of a manually-operated scanner, particularly where a large volume of items is to be processed, there are other serious shortcomings in that a fiber optic wand has limited resolution and is subject to failure. With prolonged use, the repeated flexing of the fiber optic cable often results in cable rupture.
In the more complex automatic electro-optical scanner, such as that employed in a UPC merchandising system, use is made of an oscillating scanning mechanism operating in conjunction with a monochromatic, collimated laser beam. The incident beam is focussed on the bar pattern and is swept thereacross at high speed to produce, by reflection, modulated light pulses which are intercepted by a photomultiplier tube.
In a UPC system, the items bearing code indicia are advanced across a counter window without regard to the placement of the code on the item relative to the scanning beam. Hence the laser beam must be swept across the code pattern in a manner that assures a correct reading of the code irrespective of the orientation of the code pattern. This is achieved by means of oscillatory or rotating mirrors which deflect the laser beam at high velocity in the X and Y directions to intercept the bar code at any orientation.
In this situation, in order to keep the spot in focus at any point within the acceptance volume in which the bar code is presented, it is essential that the scanning beam have a large depth of focus. And for high resolution, it is essential that the diameter of the luminous scanning spot be extremely small. A laser beam fully satisfies these requirements, but in doing so it imposes certain constraints on the scanner, which are in large measure responsible for the considerable size and high cost of existing laser beam scanners.
Because the use of a laser introduces safety problems when used outside of the laboratory in an environment exposed to the public, compliance with Federal OSHA Safety Standards and the resultant need to obtain adequate bar code contrast at relatively low laser emission levels dictate an expensive photomultiplier to achieve an acceptable signal-to-noise ratio. Moreover, since a UPC arrangement calls for high-speed laser beam scanning, the digital pulses which are generated are narrow, and this in turn entails wide-band electronics in order not to degrade the signal. Such requirements further contribute to the high cost of the scanner.
Another drawback of existing types of laser-beam bar-code scanners in which the bar-code being scanned is located in a volume above the scanner window, is that the scanner is exposed to ambient light. A narrow-band, optical filter must therefore be used which is permeable to the light derived from the laser beam and discriminates against stray light energy.
Still another drawback of flying spot scanners is that while under ideal circumstances, the flying spot will intercept the bars and produce light pulses regardless of where the point of intersection is along the bars, in actual practice, since the bars are printed and may, here and there, exhibit open gaps, the point of intersection may run across a gap and miss the bar; hence the digital signals will not correspond to the bar code.